1. Field of the Invention
The present invention relates to methods and devices for measuring optical properties such as refractive index, birefringence, and optical activity and for measuring and monitoring distances and changes thereof such as in interferometers and devices that can be used to profile surfaces such as scanning (near field) optical microscopes.
2. Brief Description of the Related Art
Many devices make use of various optical properties. For example, a camera makes use of the fact that light can be refracted, reflected and focused. Other common optical devices include microscopes and telescopes. Optical properties such as refractive indices have many other useful applications, including use in measuring a thickness of a thin film on a substrate.
The refractive index of a medium generally may be defined as the relative speed at which light moves through a material with respect to its speed in a vacuum. When light passes from a less dense medium (such as air) to a denser medium (such as water), the speed of the light wave decreases. Alternatively, when light passes from a denser medium (such as water) to a less dense medium (such as air), the speed of the wave increases. By convention, the refractive index of a vacuum is defined as having a value of 1.0. Since a vacuum is devoid of any material, refractive indices of all transparent materials are therefore greater than 1.0.
The angle of refracted light is dependant upon both the angle of incidence and the composition of the material into which it is entering. The “normal” is defined as a line perpendicular to the boundary between two substances. Light will pass into the boundary at an angle to the normal and will be refracted according to Snell's law:N1 sin(θ1)=N2 sin(θ2)where N1 and N2 represent the refractive indices of material 1 and material 2, respectively, and where θ1 and θ2 are the angles of the light traveling through materials 1 and 2 with respect to the normal.
Many methods and devices for measuring a refractive index of a medium are known. One such common device is known as a refractometer. A refractometer uses what is referred to as the “critical angle” of total reflection to measure the refractive index. When light passes through a medium of high refractive index into a medium of lower refractive index, the incident angle of the light waves becomes an important factor. If the incident angle increases past a specific value (dependant upon the refractive indices of the two media), it will reach a point where the angle is so large that no light is refracted into the medium with lower refractive index. This specific value is the “critical angle.” The critical angle may be measured by transmission (light is transmitted through a sample) or by internal reflection (light is reflected from the boundary between the sample and the prism) when a sample is placed adjacent a prism.
Other methods or devices such as those disclosed in U.S. Pat. No. 6,490,039 and U.S. Patent Application Publication Nos. US2003/0098971, US2002/0140946 and 20040023396 likewise are known.
Birefringence occurs when an optical material in the path of a beam of light causes the beam to be split into two polarization components which travel at different velocities. Birefringence is measured as the difference of indices of the refraction of the components within the material. Birefringence is an intrinsic property of many optical materials such as crystals but may be introduced by external fields applied to the material. The induced birefringence may be temporary, as when the material is strained, or the birefringence may be residual, as may happen when, for example, the material undergoes thermal stress during production of the material. The residual birefringence in an optical component affects its quality, especially when used in polarization related instruments. Linear birefringence refers to a difference in the refractive indices of two orthogonal linearly polarized light beams. Similarly, circular birefringence describes a difference in the refractive indices that right- and left-circularly polarized light experiences as it traverses the sample exhibiting circular birefringence. Birefringence (both linear and circular) may be observed as the rotation of the plane of polarization of a linearly polarized light beam.
A ring resonator can be built from standard fiber optics components, in the simplest form using only a fiber optic loop and a standard telecommunications coupler. One input of the coupler is connected to its output port closing the fiber loop. The remaining two ports form the connectors for a linear bus waveguide (fiber) which is used to couple the light evanescently into the ring structure.
Micro- and nanofabrication techniques make it possible to fabricate waveguiding structures out of a variety of materials and on a multitude of substrates. Ring resonators have been constructed with waveguides made from materials such as silica, silicon, and polymers (PMMA). Soft lithographic and micro-contact printing techniques can also be employed to manufacture waveguides.
The transmission characteristic of a ring resonator strongly depends on the frequency of the light—at specific frequencies the resonance condition for constructive/destructive interference is met when the light couples between the bus waveguide and the ring structure. The associated resonance frequencies can be determined with highest precision since the linewidth of the laser and the associated linewidth of the ring resonator is typically low. We routinely achieve resonances in a fiber-loop resonator that have sub-picometer linewidths.
The nature of the fiber or waveguide in general also accommodates modes with different polarization states. Each resonance is associated with a certain state of polarization (SOP), i.e. for instance (transverse electric) TE or (transverse magnetic) TM, and birefringence in the fiber-loop means that the different polarization states can have different resonance frequencies. The resonance frequencies can be measured by e.g. mode-hop free scanning of a tunable laser.
Ring resonators and related topics have been discussed in the following references: [1] U.S. Pat. No. 6,842,548 Loock, et al. Jan. 11, 2005; [2] A.V. Kabashin, P. I. Nikitin, Quantum Electronics 27 (1997) 653-654; [3] V.E. Kochergin, A. A. Beloglazov, M. V. Valeiko, P. I. Nikitin, Quantum Electronics 28 (1998) 444-448; [4] L. F. Stokes, M. Chodorow, H. J. Stokes, Opt. Lett. 7 (1982) 288-290; [5] F. Zhang, J. W. Y. Lit, J. Opt. Soc. Am. A. 5 (1988) 1347-1355; [6] J. E. Heebner, V. Wong, A. Schweinsberg, R. W. Boyd, D. J. Jackson, IEEE J. Quant. Elec 40 (2004) 726-730; [7] C-Y. Chao, L. J. Guo, Appl. Phys. Lett. 83 (2003) 1527-1529; [8] M. Brierley, P. Urqhart, Appl. Opt. 26 (1987) 4841-4845; [9] S. J. Petuchowski, T. G. Giallorenzi, S. K. Sheem, IEEE J. Quant. Elec. QE-17 (1981) 2168-2170; [10] L. H. Jae, M. Oh, Y. Kim, Opt. Lett. 15 (1990) 198-200; [11] D. Monzon-Hernandez, J. Villatoro, D. Talayera, D. Luna-Moreno, Appl. Optics 43 (2004) 1216-1220; and [12] A, Gonzalez-Cano et. al, Appl. Optics 44 (2005) 519-526. [13] U.S. Pat. No. 6,901,101 Frick May 31, 2005; [14] F. Vollmer, P. Fischer, Opt. Lett. 31 (2006) 453.
Various methods and apparatus for measuring optical properties, distances, etc. exist, but a need is present for the method and apparatus of the present invention, which measure optical properties with great sensitivity (via changes in the resonance frequency) and which lends itself to miniaturization as it requires no moving parts or electro-optic elements.